A number of customers has approached us with questions like “Those new MiTM
attacks against SSL/TLS, what’s their impact as for the security of our SSL VPNs
with client certificates”?
In the following we give our estimation, based on the information publicly
available as of today.
On 11/04/09 two security researchers (Marsh Ray and Steve Dispensa) published a
paper describing some
previously (presumably/hopefully) unknown MiTM attacks against SSL/TLS.
CVE-2009-3555 was assigned to the underlying vulnerabilities within SSL/TLS.
The attacks described might potentially allow an attacker to hijack an
authenticated user’s (SSL/TLS) session. In an
IETF draft
published 11/09/09 and describing a potential protocol extension intended to
mitigate the attacks the following is stated:
“SSL and TLS renegotiation are vulnerable to an attack in which the attacker
forms a TLS connection with the target server, injects content of his choice,
and then splices in a new TLS connection from a client. The server treats the
client’s initial TLS handshake as a renegotiation and thus believes that the
initial data transmitted by the attacker is from the same entity as the
subsequent client data.”
Obviously this _could_ have dramatic security impact on most SSL VPN
deployments. Still, within the research community the impact on SSL VPNs seems
unclear at the very moment.
On monday, Cisco issued a somewhat nebulous
security advisory
classifying the Cisco AnyConnect VPN Client as _not_ vulnerable (but quite
some other products). OpenVPN disposes, for some time already, of a particular
directive (tls-auth) which is regarded as an effective way to mitigate the
vulnerabilities. Other vendors (e.g. Juniper) have not yet issued any statements
or advisories at all (see also
here for an
overview of different vendors’ patch status).
This might be a good sign (“no problems there”), it might just be they’re still
researching the pieces.
After discussing the problems with other researches we expect more concrete
attack scenarios to emerge in the near future and we expect some of those future
attacks to work against _some_ SSL VPN products as well. In case you have some
SSL VPN technology in use pls contact your vendor _immediately_ asking for an
official statement on this stuff.
Sorry for not having better advise for you right now. We do not want to spread
FUD. On the other hand it might be a cautious approach to “expect the worst” in
this case. There’s vast consensus amongst researchers that this _is a big
thing_ that most people did not expect in this protocol. A first public
break-in based on the vulnerabilities has already been
reported.